1. Field of the Invention
The invention relates to a current lead-in of the coaxial type which comprises at least two conducting tubes, is carried through a container wall with a gas-tight seal, and which particularly serves for supplying a high frequency current to an induction heating coil that is used for producing a melting zone in a semiconductor rod.
2. Description of the Prior Art
The apparatus for the zone-melting of a crystalline semiconductor rod usually includes a container in which the semiconductor rod is supported, and wherein heating means in the form of an induction heating coil are disposed for heating the melting zone. The container is either evacuated, or filled with a protective gas. The heating means are supplied with alternating current from a high frequency generator. This heating current for the induction heating coil is supplied through a current lead-in, which is installed through the wall of the container. According to U.S. Pat. No. 3,688,006 this current lead-in contains two conductor tubes of different width or diameter. The tubes carry the induction heating coil and are coaxially disposed within each other due to their low inductance. An intermediate layer of elastic, cold or vulcanized silicon-rubber is used as the dielectric.
During the zone-melting of relatively thick semiconductor rods requiring relatively high power, relatively high power losses also occur in the current lead-in. Therefore during operation, small leakage points are formed between the intermediate layer of insulating material and the surface of the conducting tubes of the current lead-in, because the insulting material can easily break down by virtue of the heat load from the conducting tubes. The current intensity (amperage) in a zone-drawing operation for thick semiconductor rods can reach values of 500 to 1000 A at a frequency of approximately 3 MHz.
In German patent application No. P 28 23 611, which is also the subject of co-pending U.S. application Ser. No. 042,142 filed May 24, 1979 having the same inventor, a current lead-in is described wherein the silicon rubber is disposed around the inner conducting tube so that its layer-thickness increases conically in the direction toward the induction coil, and wherein an outer conducting tube is used which widens corresponding to the conical shape or form of the silicon rubber layer. In this way by press-fitting the two conducting tubes into each other, a seal and stop toward the outside is effected. The application of the silicon-rubber layer onto the inner conducting tube is performed in a mold which can be removed after the rubber has hardened. The casting or pouring device required for this procedure is, however, costly and the same applies for any repairs.